The invention relates to methods for wrapping a load with a film of cold-stretchable plastics material. In particular, the invention refers to a method that is usable on a wrapping machine for controlling and adjusting wrapping of a film around a load.
Known wrapping machines generally include an unwinding apparatus that supports a reel from which the plastics are unwound for wrapping around the load in such a manner as to form a series of strips or bands with a helical or helix pattern, by virtue of the combination of the movement in a vertical direction of the wrapping apparatus and of the relative rotation between the latter and the load. The latter typically includes one or more products grouped and arranged on a bench or shovel or pallet.
In wrapping machines provided with a rotating table for supporting the load, the latter is rotated around a vertical wrapping axis, whereas the unwinding apparatus is moved vertically with reciprocal movement along a fixed column.
In wrapping machines with a horizontal rotating ring or a rotating arm, the load remains static during wrapping, whereas the unwinding apparatus is moved with respect to the latter, both rotating around the vertical wrapping axis and translating along the latter. For this purpose, the unwinding apparatus is fixed to a ring or to an arm that is rotatably supported by a fixed structure of the machine and in such a manner as to rotate around the load.
In wrapping machines with a vertical ring, the load is moved horizontally through the ring, whereas the unwinding apparatus rotates with the ring around a horizontal wrapping axis.
The unwinding apparatus typically includes a pair of prestretching rollers arranged for unwinding the film from the reel and prestretching or elongating the film, and one or more deflecting or idling rollers arranged for deflecting the film towards the load. By appropriately adjusting the difference between rotation speed of the prestretching rollers, it is possible to prestretch by a defined quantity or percentage the film exiting the unwinding apparatus. By adjusting the rotation speed of the prestretching rollers it is also possible to vary the unwinding speed of the film from the reel, i.e. the speed with which the film exits the unwinding apparatus.
The unwinding apparatus generally includes an electric motor that is able to rotate one of the two prestretching rollers that acts as a master roller and drives, by a transmission/reduction unit, the other prestretching roller that acts as slave roller. In this manner, between the fast roller and the slow roller a predefined transmission ratio is set according to the desired prestretch of the film.
Unwinding apparatuses are further known to include two distinct electric motors for driving the two prestretching rollers independently.
In the operation of known wrapping machines, it is difficult to maintain a force or traction or wrapping tension (so-called “pull”) of the film around the load that is almost constant, in order to ensure a value of the wrapping or binding tension that is suitable and appropriate to the type of load to be wrapped. The need to control and limit wrapping tension to avoid film breakage is also known.
The wrapping tension varies for each wrapping revolution according to the dimensions, the shape or cross section of the load to be wrapped and the angular position between the load and the unwinding apparatus. The variations of the wrapping tension can also be considerable, especially in the case of loads with a narrow and long section or a wide and short section.
Wrapping methods are known that maintain an almost constant tension by varying the film unwinding speed, i.e. the exit speed of the film from the unwinding unit by retroactive adjustment of the rotation speed of the prestretching rollers.
For this purpose, sensors are provided (encoders, load cells) that are able to measure film tension directly or indirectly and send a corresponding signal to a control unit of the wrapping machine, the control unit being able to intervene on the motor or on the motors of the prestretching rollers to increase or decrease the rotation speed thereof.
Such retroactive control systems are, however, expensive and difficult to adjust and fine tune. Further, in the case of high performance wrapping machines, the high rotation speeds of the unwinding apparatus do not permit effective and prompt retroactive adjustment of the speed of unwinding of the film from the reel as a function of variations in film tensions.
Wrapping methods are known that control the unwinding speed of the film and/or the quantity of film to be unwound per revolution of the wrapping apparatus around the load or vice versa on the basis of the dimensions of the latter.
U.S. Pat. No. 5,123,230 discloses a wrapping method for a wrapping machine with a vertical ring that adjusts and controls the rotation speed of a film unwinding roller, in order to maintain the desired wrapping tension of the film around the load, on the basis of a sequence of values calculated by a control unit of the machine starting from the dimensions of the load.
U.S. Pat. No. 7,707,801 discloses a wrapping method for a horizontal rotating ring wrapping machine in which for each revolution of an apparatus for unwinding the film around the load a set quantity of film is calculated as a function of the perimeter of the load. The unwinding apparatus, which is fixed to the rotating ring, includes film prestretching rollers that are rotated by a belt wound on a fixed ring, the rotation of the rotating ring determining in this manner the rotation of the prestretching rollers with a defined transmission ratio. In this manner, the predefined quantity of unwound film for each revolution is independent of a rotation speed of the unwinding apparatus.
Such wrapping methods nevertheless do not ensure a satisfactory wrapping quality of the film at all rotation speeds of the unwinding unit around the load. In particular, they do not ensure constant film wrapping or binding tension around the load at all rotation speeds. Further, by unwinding a preset quantity of film for each revolution they encounter variations of the wrapping tension between the bands or strips of film wrapped with helical motion in the central portion of the load and those wrapped with circular motion in the end, lower and upper portions of the load. In order to stabilize the load and consolidate wrapping, it is in fact known to wrap the end portions with a plurality of superimposed strips of film.
If the predefined quantity of film is set to ensure correct tension of the film in the end portions, the wrapping tension in the central portion may be high and lead to an excessive narrowing of the height of the film, consequently increasing the consumption of the film. Conversely, if the wrapping tension in the central portion is correct, the wrapping tension in the end portions may be insufficient, leading to loosening of the binding.